Despite numerous advances in cancer chemotherapy, long-term survival of patients with systemic or metastatic neoplasia remains elusive. Several mechanisms of resistance result in failure of chemotherapy. One important mechanism of resistance to a diverse set of drugs has been called multidrug resistance (mdr) and is associated with the expression of a membrane protein, P-glycoprotein, which is encoded by members of the mdr gene family. We are investigating the structure, function and regulation of the rat mdr genes. Our goal is to determine the cis-acting elements critical to basal and drug-stimulated regulation of transcription of these genes and to subsequently identify the factors which bind to and regulate the mdr promoter. The signal transduction pathways which lie upstream to this transcriptional regulation are also under investigation. Towards these goals we have cloned and characterized complete cDNAs encoding two of the three rat mdr genes and have isolated a partial clone for the third rat mdr gene. To investigate the mechanisms which regulate the expression of the mdr1b and mdr2 genes, their 5' flanking regions have been isolated and characterized by sequence and functional analyses. The sequences critical to basal promoter activity of the mdr1b promoter have been identified as a transcriptional repressor in the 5' flanking region. Current efforts focus on identifying the factors which bind to and regulate this promoter under basal and drug-stimulated conditions. The P-glycoprotein encoded by the mdr2 gene is not involved in drug resistance and has recently been identified as a phospholipid transporter in the liver. We have begun to identify the sequences important to the regulation transcription of this gene. This promoter is GC rich and utilizes several transcription initiation sites which is typical of TATA-less promoters. Further investigations will delineate elements critical for mdr2 transcription.